Amnesic Shellfish Poisoning and Domoic Acid

Amnesic shellfish poisoning (ASP) is a serious illness caused by eating shellfish that have accumulated domoic acid, a natural marine biotoxin produced by certain algae. The toxin concentrates in filter-feeding shellfish such as mussels, oysters, scallops and pipis, and it is not destroyed by cooking. The main defence is monitoring and closing harvest areas when toxin levels rise, so contaminated shellfish never reach sale. Key takeaways: Amnesic shellfish poisoning (ASP) is caused by domoic acid, a natural marine biotoxin that filter-feeding shellfish accumulate from algae in the water.. Domoic acid is heat-stable, so cooking, steaming or freezing does not make contaminated shellfish safe.. The illness can cause gastrointestinal symptoms and, in severe cases, neurological effects including short-term memory loss, which gives the condition its name.. The primary control is not something a kitchen can do at the bench — it is closing harvest areas and testing shellfish before they enter the supply chain.. Businesses protect customers by buying only from approved, monitored sources, keeping supplier records and acting immediately on recalls or harvest closures.. What amnesic shellfish poisoning is Amnesic shellfish poisoning is a foodborne illness caused by eating shellfish that contain domoic acid. Unlike illnesses caused by bacteria or viruses, ASP is a biotoxin poisoning: the harm comes from a chemical the shellfish took up from its environment, not from an organism growing in the food. This distinction matters, because the usual kitchen controls for pathogens — cooking, chilling and hygiene — do not remove the risk. The name refers to the most striking symptom seen in serious cases: memory loss. Milder illness tends to bring on gastrointestinal symptoms such as nausea, vomiting, cramps and diarrhoea within hours of eating affected shellfish. In more severe cases, neurological effects can follow, which may include confusion, disorientation and, in rare and serious instances, lasting short-term memory impairment. Because the symptoms and severity vary, anyone who becomes unwell after eating shellfish should seek medical advice. There is no home remedy that neutralises the toxin, so care is supportive and focused on the symptoms. Where domoic acid comes from Domoic acid is produced by certain species of microscopic marine algae. Under particular conditions — water temperature, nutrients and season — these algae can multiply rapidly into what is often called a harmful algal bloom. During a bloom the concentration of toxin-producing algae in the water rises sharply. Filter-feeding shellfish, also called bivalve molluscs, feed by drawing large volumes of seawater across their gills and straining out plankton. When they filter water containing toxic algae, the domoic acid concentrates in their tissues. The shellfish themselves are generally unharmed and show no visible sign of contamination, which is exactly why the hazard is invisible to a buyer. The shellfish most commonly associated with biotoxin risk are the filter feeders: mussels, oysters, scallops, clams, cockles and pipis. Toxin levels in a given area rise and fall with the algae in the water, so an area that is safe one week can carry elevated toxin the next, and vice versa. Why cooking does not make it safe A central point for food handlers to understand is that domoic acid is heat-stable. Normal cooking methods — steaming, boiling, grilling or frying — do not break the toxin down to safe levels. Neither does freezing. A batch of contaminated mussels that is thoroughly cooked is still capable of causing amnesic shellfish poisoning. This sets biotoxins apart from most microbial hazards. With bacteria such as Vibrio or with norovirus, adequate cooking is a genuine control step. With ASP there is no equivalent step at the kitchen bench: once the toxin is in the shellfish, no ordinary preparation removes it. You cannot smell, taste or see it, and it does not change the appearance of the shellfish. For this reason, the safety of the shellfish has to be assured before it reaches the kitchen. The controls that matter happen upstream — in the water, at the harvest area, and through testing — not in the pan. Why closed-harvest-area controls matter Because contaminated shellfish look and taste normal and cooking will not help, the food-safety system relies on preventing harvest of affected shellfish in the first place. Shellfish-growing waters used for commercial harvest are managed under quality-assurance programs that monitor water quality and biotoxin levels on an ongoing basis. When monitoring detects that biotoxin levels are rising towards or above safe limits, the harvest area is closed. A closure stops shellfish being taken from those waters for sale until testing shows the toxin has cleared and the area can reopen. This is the single most important barrier between a harmful algal bloom and a customer's plate, and it operates well before any product reaches a business. For a food business, the practical consequence is that the safety of shellfish depends on where and how it was sourced. Shellfish from an approved, monitored source that was open at the time of harvest has passed through this control. Recreationally gathered shellfish, or product from unknown or unapproved sources, has not, and should never be served commercially. What a food business should do The most effective thing a business can do about ASP is to control its supply chain. Buy shellfish only from licensed, reputable suppliers who source from approved harvest areas covered by a shellfish quality-assurance program. This keeps the biotoxin control that happens at the water working all the way to your door. Keep records of where each batch of shellfish came from. Under the traceability expectations in the Food Standards Code, a business should be able to identify its immediate supplier for any food it receives. Good supplier records mean that if a harvest area is closed or a recall is issued, you can quickly work out whether any affected product is in your stock, and act on it. Respond promptly to recalls and public health alerts. If a harvest area is closed after you have received product, or a recall names a batch you hold, remove it from sale immediately and follow the recall instructions. Do not rely on cooking, washing or sorting to make suspect shellfish safe, because none of these steps work against domoic acid. How ASP fits with other shellfish biotoxin risks Amnesic shellfish poisoning is one of a family of shellfish biotoxin illnesses. Others include paralytic shellfish poisoning, diarrhetic shellfish poisoning and neurotoxic shellfish poisoning, each caused by different toxins produced by different algae. They share the same underlying pathway: filter-feeding shellfish concentrate a toxin from the water, and the toxin is not removed by cooking. Because the toxins differ, the symptoms and the specific safe limits differ too, but the control strategy is common to all of them. Monitoring shellfish-growing waters, testing product and closing harvest areas when levels rise protects against the whole group at once. This is why sourcing from monitored, approved areas is such a powerful safeguard — it addresses several distinct hazards through one system. For a kitchen, the takeaway is consistency: treat all filter-feeding shellfish as products whose safety was decided before they arrived. Focus your effort on approved sourcing, traceability and rapid recall response, and keep your standard controls — temperature control, hygiene and avoiding cross-contamination — running for the bacterial and viral risks that cooking and chilling can address. Checklist: Shellfish are bought only from licensed suppliers sourcing from approved, monitored harvest areas. Supplier and batch records are kept so any shellfish can be traced to its immediate supplier. Staff know that cooking, freezing and washing do not remove domoic acid from shellfish. Harvest-area closures and recall alerts relevant to your suppliers are monitored and acted on. Recreationally gathered or unknown-source shellfish are never served commercially. Suspect or recalled shellfish are withdrawn from sale immediately rather than cooked or sorted. Shellfish are kept under proper temperature control for the separate bacterial and viral risks. Common mistakes: Assuming thorough cooking or steaming makes contaminated shellfish safe — domoic acid is heat-stable. Serving recreationally gathered shellfish, which has not passed through harvest-area monitoring. Buying shellfish from unknown or unapproved sources to save money. Keeping no supplier or batch records, so a recall or closure cannot be checked against stock. Judging shellfish safety by smell, taste or appearance, which the toxin does not change. Delaying action on a harvest closure or recall notice for product already received.